1. Field of the Invention
This invention relates to the treatment of sulfide-bearing ores and, more particularly, to a novel process for removing sulfur from sulfide-bearing ores using lime in a closed system, the process proceeding in the absence of the net consumption of gaseous species and without producing gaseous, sulfur-containing pollutants.
2. The Prior Art
Historically, one of the most commonly used methods of treating sulfide-bearing ores to recover the metal values therefrom has been by conventional roasting processes wherein the ore is oxidized to produce oxides of the metal. The roasting process consumes vast amounts of oxygen and, correspondingly, produces vast quantities of sulfur dioxide, and the like, so that it is a very difficult process to isolate against the escape of gaseous, sulfur-containing pollutants. Also, while certain quantities of sulfur-bearing gases can be used for producing sulfuric acid, the acid process is known to convert less than all the gaseous sulfur compounds to sulfuric acid with a corresponding discharge or escape of gaseous sulfur residues to the atmosphere. Additionally, only limited quantities of sulfuric acid can be used in other process industries so that unlimited production of sulfuric acid is not an economically appealing prospect. In view of the increasingly stringent restrictions against the emission of sulfur-bearing pollutants, the resulting expenses involved in obtaining metal oxides from sulfide-bearing ores by conventional roasting process is becoming very high.
Researchers have proposed the use of lime (calcium oxide) to remove the sulfur dioxide produced during the roasting of copper sulfide concentrates. While the chemistry of this known process is rather complex and will not be detailed herein, the major reactions are the reaction between oxygen and, for example, chalcopyrite, which produces sulfur dioxide and the oxides of copper and iron. The sulfur dioxide reacts with lime and oxygen to form calcium sulfate. When using hydrated lime, there is an initial, small amount of water vapor released during the early stages of the process, but this water vapor is rapidly dissipated to the ambient. This process also requires that the sulfide-bearing ore and the lime be placed in close proximity in order for the lime to effectively scavenge the sulfur dioxide.
Problems are also encountered in removing relatively large quantities of thermal energy generated by the reaction inside the bulk of the mixture. Importantly, excessive temperatures can cause sintering of the mixture with resultant complications for further processing. High temperatures also promote the formation of copper ferrite (CuO.Fe.sub.2 O.sub.3) which is difficult to leach. Others have proposed that this problem can be circumvented by pelletizing the mixture so that the reaction can be controlled by the diffusion of oxygen into the pellet. Although pelletizing provides improvements in temperature control along with sulfur retention, the overall reaction is much slower because of the relatively long diffusion distances. It has been found that a substantial quantity of sulfate is formed during the process and can be undesirable depending upon the type of subsequent treatment required to obtain the desired final product. However, where copper is involved as the final product, this latter consideration is of little consequence.
One similar process is disclosed in U.S. Pat. No. 3,915,689 issued Oct. 28, 1975. This patent relates to a process wherein a copper-sulfide mineral is pelletized with lime and roasted at low temperatures. The resulting sulfur dioxide reacts with the lime to form anhydride (CaSO.sub.4).
Other researchers have studied the direct reduction of metal sulfides with hydrogen while using lime to scavenge any hydrogen sulfide generated. The general chemistry of the reaction can be expressed as follows: EQU Me.sub.x S+H.sub.2 =xMe+H.sub.2 S (1) EQU H.sub.2 S+CaO=H.sub.2 O+CaS (2)
so that the overall reaction can be expressed as follows: EQU Me.sub.x S+H.sub.2 +CaO=xMe+CaS+H.sub.2 O (3)
In each of the foregoing chemical equations the term (Me) is used to designate the metal in the respective compound.
One researcher has used lime in the reduction of molybdenite with hydrogen. Others have prepared metallic filaments of the metals, copper, nickel, cobalt, and iron, by reducing the corresponding metal sulfide with hydrogen in the presence of lime. Studies have also been conducted on the reduction of the sulfides of copper and copper/iron using hydrogen. Most of these studies were concerned with improving the unfavorable thermodynamics of the reaction between the metal sulfides and hydrogen (Equation (1), above) by providing lime as a scavenger but were not primarily concerned with the degree of removal of hydrogen sulfide from the product gas.
One researcher has observed a heavy evolution of hydrogen sulfide during the hydrogen reduction of molybdenite in mixture with lime while others reported that the gaseous reaction product was exclusively water. Recently, other researchers have proposed a process concept for producing copper fom chalcopyrite by applying the foregoing reaction scheme.
Clearly, the degree of hydrogen sulfide removal will depend upon the equilibrium concentrations of reactants, relative reaction kinetics, the relative amount of lime present, and effective diffusivities of the gases within the powder mixture. These factors, together with the overall reaction kinetics, have yet to be determined.
The primary disadvantage of the hydrogen reduction process is the use of hydrogen which is relatively expensive (whereas many metal oxides can be reduced by relatively inexpensive coke), and the fact that the sulfide and lime mixture must be pelletized to accomodate the effective removel of hydrogen sulfide. This latter factor implies (1) an additional process step, pelletizing, (2) a potential heat transfer problem, and (3) a slower reaction rate due to diffusional effects.
Another study vaguely related to the foregoing involved chalcopyrite mixed with carbon and lime. The mixture was reacted with steam to produce calcium sulfide with carbon monoxide and hydrogen as gaseous intermediates. This study was directed toward producing calcium sulfide for subsequent use in generating hydrogen sulfide. No attention was given to the metal compounds from the chalcopyrite and no kinetic measurements were made. Additionally, since the maximum conversion in terms of sulfur fixed as calcium sulfide was 40-50% in the test, it would appear likely that only the iron constituent of the chalcopyrite was converted to the oxide while the copper constituent remained relatively unchanged.
Another process for recovering sulfur from iron pyrites is disclosed in U.S. Pat. No. 1,731,516 issued Oct. 15, 1929. In this process, steam reacts with iron pyrites in the presence of an alkali to produce hydrogen sulfide. Elemental sulfur can then be obtained by reacting the hydrogen sulfide with sulfur dioxide.
The direct reduction of copper sulfide ores with hydrogen is disclosed in U.S. Pat. No. 3,701,648 issued Oct. 31, 1972. Water vapor is included with the hydrogen to accelerate removal of hydrogen sulfide and to provide additional hydrogen.
U.S. Pat. No. 3,932,170 issued Jan. 13, 1976 discloses a process for the direct reduction of sulfide ores to metallic values using hydrogen and/or carbon monoxide in the presence of a scavenging agent such as calcium oxide.
The desulfurization of iron oxide pellets using an integral carbon is disclosed in U.S. Pat. No. 3,993,472 issued Nov. 23, 1976. The pelletizing of iron ore concentrates using binders such as either calcium oxide or calcium hydroxide is disclosed in U.S. Pat. No. 3,214,263 issued Oct. 26, 1965 and U.S. Pat. No. 4,093,448 issued June 6, 1978.
A process for leaching a low-grade molybdenite with water at an elevated temperature and pressure under an oxidizing atmosphere is disclosed in U.S. Pat. No. 3,714,325 issued Jan. 30, 1973.
In summary, many of the foregoing prior art processes use lime to remove sulfur-containing gases in the processing of metal sulfides. All require a continuous supply of reactant gases and the continuous withdrawal of product or waste gases. Accordingly, there is the problem of containment and recovery of sulfur-containing gases as experienced in the other prior art processes centered around the roasting concept. Furthermore, it is considered necessary to pelletize a intimate mixture of the metal sulfide and lime to facilitate removal of the sulfur-containing gases. Pelletizing, however, hinders diffision of the reactant gases and, therefore, hinders the rate of reaction. Pelletizing is believed also to contribute to the problem of temperature control since large temperature differences can occur inside the individual pellets with accompanying complications.
In view of the foregoing, it would be an advancement in the art to provide a process for converting the metal constituent of a metal sulfide ore to a metal oxide while using lime to capture the sulfur released from the reaction. Another advancement in the art would be to provide a process for removing sulfur from a sulfide-bearing ore using gaseous intermediates, water vapor and hydrogen sulfide, to transport oxygen and sulfur between the sulfide-bearing ore and lime in the absence of a net generation or consumption of gaseous species. It would also be an advancement in the art to provide a process for treating chalcopyrite to produce bornite. Another advancement in the art would be to provide a process whereby the process can proceed in a closed system thereby precluding escape of gaseous compounds of sulfur. Such a novel process is disclosed and claimed herein.